1、Designation: E 1920 03Standard Guide forMetallographic Preparation of Thermal Sprayed Coatings1This standard is issued under the fixed designation E 1920; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A
2、 number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers recommendations for sectioning,cleaning, mounting, grinding, and polishing to reveal themicrostructural features of
3、 thermal sprayed coatings (TSCs)and the substrates to which they are applied when examinedmicroscopically. Because of the diversity of available equip-ment, the wide variety of coating and substrate combinations,and the sensitivity of these specimens to preparation technique,the existence of a serie
4、s of recommended methods for metal-lographic preparation of thermal sprayed coating specimens ishelpful. Adherence to this guide will provide practitioners withconsistent and reproducible results. Additional informationconcerning standard practices for metallographic preparationcan be found in Pract
5、ice E 3.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Refe
6、renced Documents2.1 ASTM Standards:E 3 Guide for Preparation of Metallographic Specimens2E 7 Terminology Relating to Metallography23. Terminology3.1 DefinitionsFor definitions of terms used in this guide,see Terminology E 7.3.2 Definitions of Terms Specific to This Standard:3.2.1 linear detachment,
7、na region within a TSC in whichtwo successively deposited splats of coating material have notmetallurgically bonded.3.2.2 splat, nan individual globule of thermal sprayedmaterial that has been deposited on a substrate.3.2.3 taper mount, na metallographic specimen createdby mounting a feature, typica
8、lly an interface or thin coating, ata small angle to the polishing plane, such that the visible widthexhibited by the feature is expanded.3.2.4 TSC, nthermal sprayed coating, including, but notlimited to, those formed by plasma, flame, and high velocityoxyfuel.4. Significance and Use4.1 TSCs are use
9、d in a number of critical industrial compo-nents. TSCs can be expected to contain measurable levels ofporosity and linear detachment. Accurate and consistent evalu-ation of specimens is essential to ensure the integrity of thecoating and proper adherence to the substrate.4.1.1 Example 1: By use of i
10、nappropriate metallographicmethods, the apparent amount of porosity and linear detach-ment displayed by a given specimen can be increased, byexcessive edge rounding, or decreased by smearing of materialinto voids. Therefore inaccurate levels of porosity and lineardetachment will be reported even whe
11、n the accuracy of themeasurement technique is acceptable.4.1.2 Example 2: Inconsistent metallographic preparationmethods can cause the apparent amount of voids to varyexcessively indicating a poorly controlled thermal spray pro-cess, while the use of consistent practice will regularly displaythe tru
12、e microstructure and verify the consistency of thethermal spray process.4.2 During the development of TSC procedures, metallo-graphic information is necessary to validate the efficacy of aspecific application.4.3 Cross sections are usually taken perpendicular to thelong axis of the specimen and prep
13、ared to reveal informationconcerning the following:4.3.1 Variations in structure from surface to substrate,4.3.2 The distribution of unmelted particles throughout thecoating,4.3.3 The distribution of linear detachment throughout thecoating,4.3.4 The distribution of porosity throughout the coating,4.
14、3.5 The presence of contamination within the coating,4.3.6 The thickness of the coating (top coat and bond coat,where applicable),4.3.7 The presence of interfacial contamination,4.3.8 The integrity of the interface between the coating andsubstrate, and,1This guide is under the jurisdiction of ASTM C
15、ommittee E04 on Metallographyand is the direct responsibility of Subcommittee E04.01 on Sampling, SpecimenPreparation, and Photography.Current edition approved May 10, 2003. Published July 2003. Originallyapproved in 1997. Last previous edition approved in 1997 as E 192097.2Annual Book of ASTM Stand
16、ards, Vol. 03.01.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4.3.9 The integrity of the coating microstructure with re-spect to chemistry.5. Selection of Metallographic Specimens5.1 Selection of specimens for metallographic exami
17、nationis critical if their interpretation is to be of value. Specimensmust be representative of the coating. Generally, the plane ofpolish should be normal to the coating surface so as to displaythe entire coating thickness, the substrate, and all interfaces.6. Sectioning6.1 Specimens to be mounted
18、for metallographic prepara-tion are generally not larger than 12 mm by 25 mm (0.5 by 1.0in.). The height of the mounted specimen should be no greaterthan necessary for convenient handling during polishing.6.2 In sectioning TSC specimens, care must be exercised toavoid affecting the soundness of the
19、coating and the interfacebetween the coating and the substrate. Sectioning damage ofthe coating and interface that cannot be removed by subsequentgrinding and polishing must be avoided.6.2.1 Friable, porous, or brittle coatings to be sectioned maybe vacuum impregnated with epoxy mounting compoundbef
20、ore sectioning to protect the specimen.6.2.2 Specimens should always be sectioned such that thecoating is compressed into the substrate. Sectioning techniqueswhich place the coating and interface in tension are strictly tobe avoided. Sectioning in tension may cause the coating to bepulled away from
21、the substrate or result in delamination of thecoating. During examination of the polished specimens, it islikely that this type of damage will be mistakenly interpreted.When sectioning some specimens, it may not be possible toavoid placing some areas of the TSC in tension. These areasshould be noted
22、 and not included in the evaluation of thespecimen.6.2.3 Sectioning with a hack saw will produce significantdamage to the coating and interface and is not consideredacceptable.6.2.4 Using an abrasive cutoff blade with a large particlesize abrasive produces a smoother surface than a hack saw, butstil
23、l produces coating damage that may require considerablegrinding in subsequent preparation to remove. The choice ofcutoff wheel, coolant, cutting conditions, and the type andhardness of the coating and substrate will influence the qualityof the cut surface. A poor choice of cutting conditions caneasi
24、ly overheat some TSC specimens rendering the specimensunusable for proper evaluation.6.2.5 Sectioning can be completed with minimal damage tothe cut surface by selection of one of the two followingabrasive cutoff blades:6.2.5.1 Use a diamond wafering blade with a maximumthickness of 0.63 mm (0.025 i
25、n.).6.2.5.2 Use an ultrathin aluminum oxide abrasive bladeapproximately 0.76 mm (0.030 in.) thick, which will breakdown during cutting to help reduce sectioning damage.6.2.6 Faster blade speeds, 1675 m/min. (5500 surface ft/min.) or greater, produce less coating damage. Slower bladespeeds will resul
26、t in more damage to the cut surface and are notrecommended.6.2.7 Generally, an abrasive cutoff blade selected to cut thesubstrate effectively will be the best blade for the combinationof TSC and substrate.7. Cleaning7.1 Cleaning of specimens prior to mounting is essential.All sectioning coolant shal
27、l be removed from the surface andfrom any porosity connected to the surface. Use of an organicsolvent to aid in fluid removal and thorough drying is neces-sary. Drying in an oven at low temperature (60 to 80C or 140to 176F) can accelerate this process. Any liquid residual mayimpede impregnation of p
28、orosity, as well as retard the curing ofmounting compounds causing difficulty during grinding andpolishing.7.2 Ultrasonic cleaning of TSC specimens is generally notrecommended, especially for fragile or brittle coatings, becausecoating particles may be lost during this energetic cleaningprocess. If
29、ultrasonic cleaning is found to be necessary,cleaning time should be kept to a minimum.8. Mounting8.1 General Information:8.1.1 It is always necessary to mount TSC specimens tomaintain the original structure of the specimen during grindingand polishing. Both compression mounting and castablemounting
30、 compounds are commonly used when mounting TSCspecimens. However, only castable epoxy mounting com-pounds should be used in the initial determination of the truecharacteristics of a coating before considering the use of anyother mounting compound. For some TSC specimens castableepoxy may provide the
31、 only acceptable mount. Refer to Table1 and Table 2 and Practice E 3 for characteristics of variousmounting compounds.8.1.2 By placing pairs of specimens in the same mount, timeand expense can be saved. When using this mounting method,the two coated surfaces should face each other. It may bepossible
32、 to place more than one pair of specimens in a singlemount.8.1.3 Mounting expenses may be reduced by employing thesandwich mount technique. A sandwich mount is made byusing a small amount of a better, more expensive, mountingcompound as the critical layer in contact with the specimen andTABLE 1 Char
33、acteristics of Compression Mounting CompoundsType of CompoundACharacteristicsAcrylic Cure time 10 to 15 min., optically clear, thermoplastic,good impregnation, low hardness, degraded by hotetchantsDiallyl phthalate Cure time 5 to 10 min., opaque, thermosetting, minimalshrinkage, good resistance to e
34、tchants, high hardnessEpoxy Cure time 5 to 10 min., opaque, thermosetting, minimalshrinkage, good resistance to etchants, high hardness,good impregnationPhenolic Cure time 5 to 10 min., opaque, thermosetting,shrinkage during cure can leave crevice at specimeninterface, degraded by hot etchants, mode
35、ratehardnessAThese compounds often contain filler materials, such as glass fibers or mineralparticulate.E1920032then topping off the mount with a less expensive compound.Care must be taken to use only mounting materials that arecompatible.8.1.4 Taper mounting may be useful for examination of theinte
36、rfaces between bond coating and top coating, as well asbetween coating and substrate.8.1.5 All components of mounting compounds includingresins and catalysts, as well as any dyes or colorants, should behandled in accordance with the manufacturers instructions.Material Safety Data Sheets are availabl
37、e from the manufac-turer.8.2 Compression Mounting Compounds:8.2.1 Curing of compression mounting compounds is ac-complished by the use of a heated press designed for anddedicated to metallographic mounting. Compression mountingcompounds require the use of heat (140 to 150C or 284 to302F) and pressur
38、e (up to 29 MPa or 4200 psi) to properlycure the mount. Therefore, mounting compounds of this typeshould only be used to mount dense, nonfriable coatings withsubstrates a minimum of 1.5 mm (0.060 in.) thick.8.2.2 Compression mounts should be cooled to below 40C(104F) while under pressure, preferably
39、 by use of a watercooled mounting press, to prevent formation of a crevicebetween the mounting compound and the specimen as theycontract unevenly during cooling.8.3 Castable Mounting Compounds:8.3.1 Castable mounting compounds may be used for allTSC specimens, but are particularly useful for porous
40、speci-mens. These mounting compounds are usually prepared bymixing two components just prior to use. The specimen isplaced in a mold, usually a cup or ringform, into which thecompound is poured.8.3.2 Vacuum impregnation of porous specimens with anepoxy resin is required. The specimen and mold are pu
41、t into avacuum chamber which will allow the castable epoxy to bepoured into the mold after the chamber has been evacuated.Vacuum pressure should be in the range of 630 to 760 mm (25to 30 in.) of mercury. The vacuum should be maintained forabout two to ten min before allowing air into the chamber. Lo
42、wviscosity epoxy resins are recommended for best results duringvacuum infiltration and subsequent preparation of TSC speci-mens.8.3.3 Addition of fluorescent dyes (1) or other agents, suchas Rhodamine B (2), that produce color response upon fluo-rescent excitation or with crossed polarized illuminat
43、ion,respectively, to the castable compound during mixing will aidin the identification of impregnated porosity during micro-scopical examination.8.3.4 Care should be used when handling castable mountingcompounds. Suppliers instructions for the use and handling ofthese materials should be followed.9.
44、 Grinding and Polishing9.1 Due to the many different types of TSCs and substratematerials, grinding and polishing sequences will vary. Properchoice of polishing surface, lubricant, abrasive type and size,time, force, polishing wheel speed and relative rotation willproduce accurate and consistent res
45、ults. Some TSCs mayrequire specialized preparation techniques. However, manycoatings can be prepared on automatic and semiautomaticpolishing equipment by use of one of the three procedureslisted in Table 3, Table 4, and Table 5.9.1.1 Use of automatic and semiautomatic polishing equip-ment has been f
46、ound to reduce inconsistency in polishedspecimens and is therefore required in the metallographicpreparation for TSC specimens. These machines provide re-producibility in wheel speed, specimen position on the wheel,applied pressure, relative rotation, and polishing time. Manualgrinding and polishing
47、 is not recommended because of theinherent inconsistency and the sensitivity of these specimens topreparation technique. (3,4,5,6)9.1.2 Most of the equipment for automatic and semiauto-matic grinding and polishing move the specimen around arotating wheel covered with abrasive so that the specimenfol
48、lows an epicycloidal path. The scratch pattern consists ofrandom arcs, and upon inspection, should be uniform acrossthe entire specimen surface before proceeding to the next stepin the preparation sequence.9.1.3 Each of the methods outlined in Table 3, Table 4, andTable 5 requires the following four
49、 basic preparation steps:9.1.3.1 Planar grinding is used to remove large amounts ofmaterial rapidly and to bring the group of specimens in theholder into coplanarity. This step is necessary to avoid asituation where one or more specimens in the holder do notcontact the abrasive while the specimens in contact receiveexcessive pressure. Planar grinding of specimens also shouldremove the layer of deformed material resulting from theprevious sectioning operation.9.1.3.2 Fine grinding removes disturbed material from theplanar grinding step while introducing a layer of distur
